(i) Field of the Invention
The present invention relates to a spark plug cap which is fitted into the upper part of a spark plug with a built-in pressure sensor mounted on an internal combustion engine, and which has a conductive path for applying an igniting high voltage to the spark plug, and further to a method of fitting a spark plug with a built-in pressure sensor.
(ii) Description of the Related Art
In a conventional internal combustion engine, in order to generally control an engine operation, the operation state of the internal combustion engine is detected. For example, by detecting the pressure (cylinder inner pressure) in a combustion chamber, the miss fire or knocking can be sensed, and the fuel consumption ratio can be enhanced. Moreover, known as a pressure sensor for measuring this cylinder inner pressure is a pressure sensor which is disposed on a spark plug attachment seat and which detects the cylinder inner pressure by the variation of a spark plug tightening load and outputs an electric signal from a sensor output cable in accordance with the detected pressure.
However, in order to introduce the electric signal from this pressure sensor to external apparatuses such as an electronic controller, it is necessary to secure a space for laying the sensor output cable upward from below a metal shell of the spark plug on the outer peripheral part of the metal shell. Particularly, when a coil is disposed in a spark plug hole, there is little clearance between the inner surface of the spark plug hole and the outer surface of the coil. Therefore, a channel for passing the sensor output cable has to be formed in the spark plug hole, a spark plug hole shape becomes complicated, and there arises a problem that the cost increases. On the other hand, when the entire diameter of the spark plug hole is enlarged so as to pass the sensor output cable, the spark plug hole shape does not become complicated. However, when the attachment angles of suction and exhaust valves are designed for a purpose of enhancing the performance of the internal combustion engine, it is desirable to minimize the diameter of the spark plug hole positioned between the suction valve and the exhaust valve.
In order to satisfy such request, a spark plug with a built-in pressure sensor is proposed as a pressure measuring apparatus (see Japanese Patent Application Laid-Open No. H6-290853, U.S. Pat. No. 5,955,826, EP609787). In the spark plug with a built-in pressure sensor, a piezoelectric element is disposed in the mounting base of a metal shell, the cylinder inner pressure of the internal combustion engine is detected by detecting the change of a tightening load of the spark plug, and a sensor output cable for outputting the electric signal in accordance with the detected pressure is guided from the upper part of the metal shell. Consequently, the sensor output cable can be extracted from the spark plug hole without enlarging the diameter of the spark plug hole. This can secure a wide space for disposing the mechanism of the suction and exhaust valves in a cylinder head, and it is therefore possible to design the attachment angles of the suction and exhaust valves for a purpose of enhancing the performance of the internal combustion engine.
Furthermore, the spark plug with a built-in pressure sensor is fitted into the bottom part of the spark plug hole of an internal combustion engine via a male screw (screw groove) disposed in the metal shell, and a gap between a center electrode and a ground electrode for generating sparks is disposed in a cylinder. Moreover, when the sensor output cable is extracted from the spark plug hole, and connected to the external apparatuses such as the electronic controller, the cylinder inner pressure detected by the spark plug is used for detecting the operation state of the internal combustion engine, for example, in the internal combustion engine operation control processing executed by the electronic controller.
However, the spark plug with a built-in pressure sensor is not easily fitted into the internal combustion engine because of the presence of the sensor output cable. Specifically, since the spark plug is fitted into the internal combustion engine by the male screw disposed on the metal shell, there arises a problem that the sensor output cable is wound around fastening tools such as a plug wrench during fitting operation.
In order to prevent the sensor output cable from being wound, it is necessary to move the sensor output cable with the rotation of the spark plug during the spark plug fitting operation, which requires much labor and makes difficult the spark plug fitting operation.
On the other hand, the plug wrench as the fastening tool for the spark plug with a built-in pressure sensor has an engagement part with the spark plug, which is provided with a groove for passing the sensor output cable extended from the metal shell. However, when the fastening operation is performed in a float state without noticing that the engagement part of the plug wrench incompletely engages with the hexagonal part of the spark plug, the sensor output cable is sometimes caught in the plug wrench groove by mistake When the plug wrench is rotated in this state, the sensor output cable is pulled, and falls from the metal shell.
The falling of the sensor output cable occurs not only during the fitting of the spark plug but also during the detaching of the spark plug.
Moreover, in order to depress the noise influence from the outside, the sensor output cable may be covered with a mesh (shield lines) formed of stainless steel or another metal. For the spark plug with a built-in pressure sensor provided with the sensor output cable covered with the metal mesh, during the fastening operation for the attachment to the internal combustion engine, when the sensor output cable contacts the spark plug hole, the metal mesh scrapes and damages the inner wall surface of the spark plug hole in some cases. Moreover, when the scrapings resulting from the scraping of the spark plug hole inner surface are caught between the spark plug and the cylinder head, the screw groove may be damaged or the air tightness of the combustion chamber may be deteriorated. Additionally, when the shield lines of the sensor output cable are ruptured by the friction of the sensor output cable with the spark plug hole inner wall surface, the effect of reducing the influence of the noises such as an ignition noise from the outside is deteriorated.
Additionally, a spark plug cap connected to a high voltage terminal is fitted into the spark plug with a built-in pressure sensor mounted on the internal combustion engine in order to connect the spark plug terminal electrode to the high voltage terminal of an ignition coil. However, when the spark plug cap is mounted on the spark plug positioned in the bottom part of the spark plug hole, since it becomes difficult to insert the spark plug cap into the spark plug hole, the spark plug hole is narrow and the sensor output cable offers an obstruction.
Wherefore, an object of the present invention is to provide a spark plug cap which facilitates the detaching/fitting operation of a spark plug with a built-in pressure sensor with respect to an internal combustion engine, fails to scrape a spark plug hole inner surface, and which is easily fitted into the spark plug.
Another object of the present invention is to provide a method of easily fitting a spark plug with a built-in pressure sensor.
To attain this and other objects, according to the present invention, there is provided a cylindrical spark plug cap which is fitted into a spark plug with a built-in pressure sensor and which includes a conductive path for applying an igniting high voltage to the spark plug and an insulating member for insulating the conductive path. The spark plug cap is provided with: a cylindrical member including a first engagement part for engaging with a fastening tool on one end, and a second engagement part for engaging with a metal shell to fasten the spark plug on the other end; and a cable engaging part for extending a sensor output cable through the cylindrical member from an end on a first engagement part side to an end on a second engagement part side.
Moreover, there is provided a method of using the spark plug cap and fitting the spark plug with a built-in pressure sensor into a spark plug hole in an internal combustion engine. The method includes the steps of: engaging the second engagement part of the cylindrical member with the spark plug; engaging the sensor output cable with the cable engaging part of the cylindrical member; fitting the fastening tool in the first engagement part of the cylindrical member; rotating the fastening tool while the sensor output cable engages with the cable engaging part; and screwing the spark plug to the spark plug hole of the internal combustion engine.
When the spark plug cap is used, during the fitting of the spark plug into the internal combustion engine, the sensor output cable rotates around the cylindrical member simultaneously with the rotation of the plug wrench, the cable fails to be wound around the plug wrench.
By providing the cable engaging part with a groove which has a sectional shape for substantially accommodating the sensor output cable, the metal mesh covering the sensor output cable fails to scrape the inner wall defining the spark plug hole, the screw groove is prevented from being damaged by the scrapings caught between the spark plug and the cylinder head, and the air tightness in the combustion chamber can be prevented from being deteriorated.
Moreover, in the above-described constitution, during the spark plug fitting operation in a state in which the spark plug cap is fitted into the upper part of the spark plug with a built-in pressure sensor, the first engagement part can be positioned in a spark plug hole shallow area or an area outside the spark plug hole, and the plug wrench can therefore securely engage with the first engagement part. This avoids the disadvantage caused by the fastening operation while the plug wrench is incompletely fitted, and prevents the sensor output cable from being caught in the groove of the plug wrench and from falling off.
The spark plug cap is provided with the conductive path for applying the igniting high voltage to the spark plug disposed in the bottom part of the spark plug hole. Therefore, even after the spark plug fitting operation, the spark plug cap is disposed in the spark plug hole while the spark plug cap is fitted into the spark plug with a built-in pressure sensor. Moreover, even during the detaching of the spark plug, similarly as during the fitting, the spark plug can be rotated while the sensor output cable engages with the cable engaging part of the spark plug cap. Therefore, even during the detaching of the spark plug, the sensor output cable is prevented from being caught in the cable passing groove disposed in the engaging part of the spark plug fastening tool and from falling off.
Furthermore, the spark plug cap of the present invention is fitted into the spark plug with a built-in pressure sensor before mounted on the internal combustion engine. Therefore, different from a case in which the spark plug is first mounted on the internal combustion engine and the plug cap is then fitted into the spark plug in the spark plug hole, the sensor output cable offers no obstruction, and the spark plug cap fitting operation is facilitated.